1. Field of the Invention
The invention relates generally to image forming apparatuses, and more specifically to a document feeding and reading unit and an image forming apparatus capable of reducing errors in magnification copying when a change in the diameter of drive motors results from heating.
2. Discussion of the Background
In increasingly versatile image processing machines such as duplication apparatuses and other similar apparatuses, several improvements have been made on image qualities. For example, it is of great importance to improve the accuracy in document reading during an image reproduction process.
Among various units incorporated into a variety of image processing machines, such as image forming apparatuses, facsimile apparatuses, and multi-functional image processing machines having copy and facsimile capabilities, a document feeding and reading unit is included as a requisite part.
Since the document feeding and reading unit in an image forming apparatus is generally located in the upper portion of the apparatus for the sake of the copying operation, the unit is likely to be heated through copy operations by a fixing unit and copy sheets transported in the apparatus.
When the document feeding and reading unit is heated, several problems have arisen in the structure of previous feeding and reading units. For example, when the materials for forming rollers in use for feeding a document sheet have relatively large linear thermal expansion coefficients, and the rollers are thereby affected by appreciable thermal expansion, the change in roller diameter by heating may result in a non-reproducible document holding position depending on changing temperatures. As a result, the document position may be irregular with time and not reproducible for different iterations in the document feeding and reading unit, thereby worsening the accuracy in document reading.
For rollers formed of rubber materials, in particular, their linear thermal expansion coefficients are generally in the range of (1˜4)×10−4/° C., which is larger than those for metals by approximately one order of magnitude. Therefore, worsening in the accuracy of document reading is likely to arise in a document feeding and reading unit using rubber rollers for document feeding.
This difficulty may seemingly be avoided by providing a temperature sensor as disclosed in Japanese Laid-Open Patent Application No. 57-211167. That is, in a document feeding and reading unit configured to feed a document sheet automatically from a document supply unit onto a document platen and to discharge the document sheet also automatically upon completing the document reading, the document holding position on the platen is controlled by measuring the temperature with a temperature sensor disposed at a document supply unit and by controlling the number of drive pulses applied to a document transport roller depending on the temperature measured.